Electron emission source with sharply defined emitting area



ELECTRON EMISSION SOURCE WITH SHARPLY DEFINED EMITTING AREA Filed March 15, 1966 [r7 1/227 25 0 ms. l l/f/filam C. Hag/72$, Dona/a C. Peroaky,

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United States Patent 3,436,584 ELECTRON EMISSION SOURCE WITH SHARPLY DEFINED EMITTING AREA William C. Hughes, Scotia, and Donald C. Peroutky,

Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Mar. 15, 1966, Ser. No. 534,345 Int. Cl. H01j 19/06, 19/10 US. Cl. 313346 5 Claims ABSTRACT 0F THE DISCLOSURE The present invention relates to an improved electron source for use in an electron gun and more particularly to a source configuration and arrangement which will minimize electron emission from all surfaces of the source other than the desired emitting surface.

Among the growing number of uses to which electron guns have been put is that of recording information on thermoplastic or photographic films by directing a high density electron beam over the film surface. Depending on the type of film used as a target, the impinging beam may cause tactual, electrostatic, or visual changes in the film surface. To assure accurate recording of information, the electron beam should be sharply defined. One method of accomplishing this is to utilize a series of focusing or shaping electrodes located along the path of the beam. Another method is to limit the emitting area of the electron source so that the beam will be relatively small as it leaves the source. Even when the second method is used, some beam focusing system must be used, although it need not be as elaborate as Where the emitting area of the source is not limited. The present invention is drawn to an improved configuration for use in the latter type of system. However, it is clear that the electron source may also be employed in any of the other well-known uses of electron guns.

While its stability, low evaporation rate, and resistance to contamination make lanthanum boride a desirable source material for electron guns used in recording processes, certain other of its characteristics have militated against its use. For one thing, unless material holders made from the common refractory metals are carburized or boronized, the boron atoms diffuse into the metal, thus destroying the electron emitting capabilities of the source. If the metal is treated by carburizing or boronizing it, the result is a holder far too brittle to withstand severe treatment. It has been found that the refractory metal rhenium can be used as a holder for the lanthanum boride material without encountering the boron diffusion problem to any great extent. However, a few molecules from the lanthanum boride material still migrate across the rhenium material adjacent to the emitting surface. Since this area is normally exposed to an accelerating electric field, electrons from these molecules are emitted from the source along with the electrons from the desired emitting area. The end result is a poorly defined beam having a diameter larger than desired.

It is one object of this invention to provide an improved configuration for an electron source which will ice minimize stray emission from source surfaces other than the desired emitting surface.

It is another object of the present invention to provide an improved configuration for an electron source which may be used in combination with beam focusing elements in order to provide a sharply defined beam of a desired slze.

It is still another object of this invention to provide, in one embodiment, an improved configuration especially useful with rhenium holder and lanthanum boride source materials.

To fulfill these and other objects of the invention, there is provided an electron source which includes a sintered block of electron emitting material having a relatively large body portion with a small neck portion projecting from one end thereof. The sintered block is enclosed within a metal holder having the same general configuration as the block itself. Only the extreme end of the neck portion of the sintered block is exposed and only this end portion is subjected to an accelerating electrostatic field. The remainder of the holder is not subjected to the accelerating field and there is little or no tendency to draw off electrons in any electron emitting material that may have migrated across the holder surface.

While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the structure, advantages, and further objects of the invention may be more readily ascertained from the following detailed description when read in conjunction with the accompanying drawing in which:

FIGURE 1 is a sectioned side view of an electron source representing one embodiment of the present invention.

'Referring now to FIG. 1, there Will be seen an electron source including a tubular metal holder 10 having a first end wall 11, a second end wall 12 generally parallel to the first end wall, and a continuous side wall 13 joining the end Walls 11 and 12. The first end wall 11 includes a neck portion 14 projecting perpendicularly from its surface. The inside diameter of the neck portion 14 is approximately 10 mils, the diameter of the desired emitting surface. 7

Preferably, the side wall 13 and the neck portion 14 are cylindrical in nature. However, other shapes of side walls and neck portions are intended to be within the scope of the invention. Furthermore, it is not necessary that the remainder of the holder 10 be the same shape as the neck portion 14 since the holder portion defined by end walls 11 and 12 and side wall 13 plays no part in the beam focusing process, but instead serves only as a reservoir of material. Thus, it need not be cylindrical, but can be frustoconical or pyramidal, for example.

While the configuration is especially useful where rhenium is to be used as the material for holder 10, other types of holder materials may be used provided the source material is compatible with the holder material chosen. For example, the holder 10 could be made from common refractory metals such as molybdenum or tantalum if the source material was of the type known as a barium dispenser material.

The interior of the holder 10 is filled with a sintered block 15 of material which Will yield a beam of electrons from its emitting surface 16 at the mouth of the neck portion 14 under the following conditions. First, the emitting surface 16 must be exposed to an accelerating potential. Second, it must be heated to a relatively high temperature. If materials known as barium dispenser materials are to be used, the sintered block must be heated to a temperature of around 1100 to 1200" C. If lanthanum boride is to be used, it must be heated to approximately 3 1500*l600 C. In the embodiment shown in FIG. 1, a sintered block 15 is heated by passing a current through the holder through conductive supports 17 and 18, which are connected to a suitable power supply (not shown).

The emitting surface 16 of the sintered block is approximately level with the bottom edge 19 of a beam shaping electrode 20 which is kept at or near the same potential as is the electron source itself. The beam shaping electrode 20 provides one terminal for an electrostatic field which may be formed between it and an accelerating electrode (not shown) that is maintained at a much higher potential. Since the opening in the beam forming electrode 20 is only slightly larger than the outside diameter of the neck portion 14 of the holder 10, the electrostatic force lines of the field affect only the emitting surface 16 of the sintered block 15.

Since only the desired emitting surface is exposed to the accelerating potentials of the electrostatic field, there is little tendency for any electrons transmitted from other areas of the metal holder to be accelerated along with the beam. Moreover, the previously enumerated materials which may comprise holder 10 have low electron emission characteristics at the operating temperature ranges for the electron emitting material, and the beam forming electrode 20 acts as a physical barrier to prevent the passage of stray electrons in the direction of the beam.

In order to assure that the emitting surface 16 is level with the end of the neck portion 14, the material 15 is packed into the holder in the following manner. During the manufacture of the source, the end wall 12 of the holder 10 is not in place. The lanthanum boride is packed into the holder end in the form of a slurry of lanthanum boride powder and amylacetate under a pressure of 30,000 p.s.i. To solidify the slurry, the source material in the holder is vacuum fired at 1750 C. End wall 12 is added to the holder either before or after the vacuum firing step and is secured in place by electron beam or spot resistance welding it to the side wall 13 at the juncture 21 of the two parts.

While there has been described at present what is regarded as a preferred embodiment of the present invention, modifications and variations may occur to those skilled in the art. Therefore, it is intended that the appended claims shall cover all such modifications and variations as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electron source for an electric discharge device comprising a body member fabricated from a material having low electron emission characteristics at a given operating temperature range, said body member having a first closed end and a second partially closed end with a projecting neck portion having outer dimensions smaller than the outer dimensions of said body member and an opening therethrough to the interior of said body member; and an electron emitting material being electron emissive at the given operating temperature range, said electron emitting material being contained within said body member and having a projection coextensive with the opening through said projecting neck portion.

2. An electron source for an electric discharge device comprising: a sintered block of electron emitting material being electron emissive at a given operating temperature range comprising a relatively large body portion having first and second end surfaces of the same general configuration joined by side walls extending substantially at right angles to said end surfaces and a small neck portion projecting from said first end surface and terminating at a surface substantially parallel to the plane of said first and second end surfaces, said neck portion having side walls extending substantially parallel to the side walls of said body portion; and a holder for said sintered block including connected wall portions covering the first and second end surfaces, said side walls of said body portion, and said side walls of said neck portion, said holder being fabricated from a material having low electron emission characteristics at the given operating temperature range.

3. An electron source as recited in claim 1 wherein said body member and said neck portion are formed with cylindrical side walls.

4. An electron source as recited in claim 2 wherein said electron emitting material comprises lanthanum boride and said holder is comprised of rhenium.

5. An electron source as recited in claim 2 in further combination with electron beam shaping device comprising an annular metal body having an aperture therethrough aligned with said emitting surface and defined by first and second intersecting wall portions, said aperture being of the same general dimensions as said emitting surface.

References Cited UNITED STATES PATENTS 3,013,171 12/1961 Beck 313-346 X 3,270,239 8/1966 Lozier et al 313346 X 3,312,856 4/1967 Lafferty et al 313337 X FOREIGN PATENTS 1,186,953 2/1965 Germany.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

US. Cl. X.R. 3l3-345, 311, 336 

